This proposal requests funding for the purchase of a Super Argus Positron Emission Tomography-Magnetic Resonance (PET-MR) imaging system made by Sedecal Inc. for preclinical small animal multimodal imaging to support the research efforts in the Department of Radiology at the Massachusetts General Hospital (MGH). The device is the first commercial preclinical microPET-MR system available in the United States, and has fully integrated state-of-the-art high-resolution PET and state-of-the-art high-resolution MR capabilities. The system transaxial field of view is 68 mm for both components allowing both mouse and rat imaging. The PET acquisition has sub-millimeter spatial resolution, the highest sensitivity per slice of any commercial system, and high uniformity. Optimized algorithms result in rapid reconstruction of less than 2 min for 3D OSEM and less than 5 sec for filtered back projection. The MR acquisition module is based on a novel cryogen-free design, superconducting 3T magnet, with magnetic gradient strength of 125mT/m. The system has an integrated graphical user interface for both PET and MR acquisitions. This scanner will significantly enhance the research efforts of a diverse group of NIH funded investigators. The system will provide high-resolution multimodal imaging capabilities for non-invasive serial evaluation of MR agent and PET radiotracer distribution to facilitate research in new multimodal molecular tracers; provide high resolution correlative anatomic MR images for early assessment of response to novel therapies evaluated with changes in radiotracer uptake measured at the sub-organ level; and provide a platform for testing novel multimodality kinetic modeling and improved image reconstruction algorithms. The system has three major advantages over separate PET and MR devices for rodent imaging. First, the combined device can achieve temporal resolution for both modalities not attainable if animals are required to be moved between imaging systems; this is vital for both multimodal agent development and multimodal kinetic modeling. Second, spatial co-localization is greatly improved with a combined system, which is vital for brain receptor studies and assessment of genetically engineered mouse models of disease. Third, throughput is greatly enhanced without the need to move animals between systems, improving data collection. The application contains projects that comprise a broad base of users representing various disciplines (radiology/nuclear medicine, chemistry/radiochemistry, oncology, neurology, genetically engineered mouse modeling, kinetic modeling, image reconstruction, therapeutic stem cell delivery, and tissue engineering) and NIH funding sources (NCI, NIBIB, NHLBI, NINDS, NIDA, NIMH, NIAMS).The necessary administrative and technical direction is already in place to ensure efficient utilization of this instrumentation. This will be the first such system, commercial or otherwise, in the greater Boston area. It will provide a platform to test developed agents and imaging paradigms that will complement the human PET-MR efforts at MGH, and will be major asset to a large community of NIH funded investigators in the region.